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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: CIE alevel on October 23, 2009, 09:35:11 AM

Title: Equlibria - partial pressure
Post by: CIE alevel on October 23, 2009, 09:35:11 AM

Hi,
**Question:
In an experiment , b mole of hydrogen iodide were put into a sealed vessel under pressure p . At equilibrium, x mole of the hydrogen iodide had dissociated , the reaction being represented by the following equation ,

2HI(g) <---> H2(g) + I2(g)

Express Kp in terms of b and x .

**I know,
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Flatex.codecogs.com%2Fgif.latex%3FK_p%2520%3D%2520%5Cfrac%2520%7Bp%28H_2%29%5Ctimes%2520p%28I_2%29%7D%7Bp%28HI%29%5E2%7D&hash=d8bb79e498a58a8fa765abb599ef392de24ed21c)

Will there be p, pressure (1 of terms from the question) in the above Kp equation?
If so then, don't we express Kp in terms of b, x and p?
What is done with "sealed vessel under pressure p"??

What do i equate : (https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Flatex.codecogs.com%2Fgif.latex%3Fp%28H_2%29%2C%2520p%28I_2%29%5C%3A%2520and%5C%3A%2520p%28HI%29&hash=caa5c18275308809adf4a74374aa6e866cf6035f)

Let me know asap. Thanks :-\

Title: Re: Equlibria - partial pressure
Post by: typhoon2028 on October 23, 2009, 10:40:10 AM

You need to convert to partial pressures, and also mole fractions.

In this problem you start with total moles of HI = b
x amount of HI reacts to H2 & I2.

Resultant moles of HI = (b-x)
x = moles of H2 + moles of I2
moles of I2 = moles H2
H2 moles = x/2
I2 moles = x/2

Total moles never changes, so total moles = b.

Kp = P_H2 * P_I2 / (P_HI)²

P_H2 = partial pressure of H2, which equals X_H2 * P_total

X_H2 = mole fraction of H2, which equals xb/2

If you do all the algebra you will see the Ptotal terms will cancel out and you are left with an equation in terms of x and b.